Hello All,
I want to learn Microcontroller from basic and step by step online.
So, where I can find reliable source to get started? Also, which I should start first? I mean as far I heard there are several categories like AVR, 8051 etc.
Among them which kind is the most appropriate for today's world?

I've never used the Arduino Uno that mcgyvr suggested, but I know that it's a very good learning tool.

I jumped right in with PICs using the PICKit 3 and their demo board. It takes some figuring out to get everything set up properly, but once you do figure it out, there are soooooo many possibilities. The PICKit 3 with demo board can be found here.

If your a newb go arduino, it runs on whats calls Stupid Simple C, which is basically a mix between Basic and C, its really easy and pinouts are already done, not to mention how much there is for the lil thing. Once your comfortable and outgrowing the basic arduino I recommend a PIC or ATMEL AVR chip to work with, possible a LPC if you need even more power.

If I had it to do over, I would have gone with Arduino. Instead I chose Picaxe and learned that system but it is not as well recognized and supported as the Arduino system by a country mile. I learned Picaxe and did a lot of simple to fairly complex tasks, but the system is slow and easilly stalled due to timing and the use of part of the memory to compile the program into a machine readable and executable form. This is just my .02, but that's how I started out and since then I have decided to go to Microchip and learn C and eventually C++. Good luck and above all don't ever give up. Your miracle will happen if you stick with it.
Wheelchair Bob

If I had it to do over, I would have gone with Arduino. Instead I chose Picaxe and learned that system but it is not as well recognized and supported as the Arduino system by a country mile.

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I haven;t used the picaxe but I've studied up on it. Just seemed to expensive for what you got and it was just a rebranded pic that was limited. Also didn't like the programming interface. Arduino's nice, anything with a good branded dev kit will be nice.

Other good thing about arduino is it's cult following of noobs, and the pursuant plethora of already asked questions with answers. Their (arduino) forum is abuzz with askers & answerers. Lots of tech support, from peers. It's open source so the DIY crowd is all over it.